Process for removing dust from a dust-laden gas using a gas-permeable filter element arranged in a container

ABSTRACT

A method of removing dust from a dust-laden gas using gas-permeable filter elements that are disposed in tanks, with the gas flowing through the filter elements from the outside to the inside thereof and depositing dust on the outer surfaces thereof. Deposited dust is removed from the filter elements by pressurized gas cleaning pulses applied to the interior of the filter elements. A plurality of tanks are supplied in parallel with partial streams of the dust-laden gas. The ratios of the pressure differentials that occur across the filter elements to the flow rates of the individual partial streams, which ratios increase with time during the formation of a dust layer, are determined. When the ratio of one of the tanks reaches an experimentally determined limiting value, this tank and all of the other tanks are simultaneously cleaned.

BACKGROUND OF THE INVENTION

The present invention relates to a method of removing dust from adust-laden gas using gas-permeable filter elements that are disposed inat least one tank, with the gas flowing through the filter elements fromthe outside to the inside thereof and depositing the dust on the outersurfaces of the filter elements, and with the deposited dust beingremoved from the filter elements by compressed or pressurized gas pulsesthat are applied to the interior of the filter elements, whereby the gaspulses are released at least as a function of a pressure differentialthat is measured across the filter elements.

Such a method is used, for example, for cleaning dust-laden flue gasesfrom pressurized or atmospheric fluidized bed reactors. In thisconnection, customarily a plurality of tanks are supplied in parallelwith the flue gas that is to be cleaned. Since only the pressuredifferential across the filter cartridges is determined and the cleaningpulses are released by the pressure differential as a function ofreaching a prescribed limiting value, a cleaning or dust-removal is alsoeffected at pressure differentials that are not based upon the formationof a sufficiently thick layer upon the filter elements, but rather uponchanges in the flow rate. Thus, the cleaning pulses are also releasedwhen no layer is present or where the layer is to thin.

It is an object of the present invention to provide a method with whichthe release of the cleaning pressure pulses is essentially effected onlyas a function of the formed dust layers.

SUMMARY OF THE INVENTION

This object is realized in that for the tank, in addition to thepressure differential over the filter elements, the flow rate of the gasthat is to be cleaned is determined, and the cleaning pulse is releasedwhen the ratio of the pressure differential to the flow rate, whichratio increases with time during the formation of the dust layer,reaches t least one experimentally determined lower limiting value.

The pressure drop caused by the gas flowing over the filter elements canbe determined in conformity with the following Darcy formula: ##EQU1##where: V is the flow rate of the gas that is to be cleaned,

η the viscosity of the gas that is to be cleaned,

L the thickness of the dust layer+the thickness of the filter wall,

A_(F) the surface area of the filter,

D_(S) a material constant of the dust.

This equation can be found, for example, in "Chemical Engineers'Handbook", 5th Ed. (Intern. Students Ed.), McGraw-Hill (1973) pages5-54.

The flow rate can be measured, for example, by introducing a restrictorin the cleaned gas stream and by determining the pressure differentialacross the restrictor. In a manner known per se, the flow rate isproportional to the square root of the pressure differential measuredacross the restrictor.

Thus, from equation (1) the ratio ##EQU2## can be determined, and can becompared with an experimentally determined fixed value.

This ratio increases during the cleaning operation, and it is possibleto experimentally determine the limiting value at which it is expedientto release the cleaning pulse. The method of the present inventionprevents cleaning pulses from being released merely by changes in flowrate, or where the thickness of the layer is still too small.

The intensity of the cleaning pulse is essentially determined by thecohesive force of the dust particles that form the layer, and by thepressure differential that is formed across the filter elements and isdirected from the outside toward the inside. In this connection, it isadvantageous if as a function of the pressure differential that isobtained across the filter elements, the pressure of the suppliedcleaning gas be set in such a way that the intensity of the cleaningpulses is adequate for cleaning or dust removal. The ratio, and hencethe intensity, are experimentally determined.

If the quantity of the dust-laden gas that is to be cleaned makes itnecessary to use a plurality of tanks, it is advantageous that with thisplurality of tanks, which are supplied in parallel with partial streamsof the gas that is to be cleaned, the ratios of the pressuredifferentials across the filter elements of the individual tanks to theflow rates of the individual partial streams, which ratios increase withtime during the formation of the dust layer, be determined and when theratio at one of the tanks reaches an experimentally determined limitingvalue, this tank and all of the other tanks are simultaneously cleaned.

With the use of a plurality of tanks that are supplied in common, thepressure differential that is obtained across the individual tanks isdetermined by the pressure differential across the filter elementsdisposed in the filter tank and by the pressure differential that isassociated with the individual tanks as a result of the distributors,collectors, and connecting channels that are required. In other words,as a consequence of the varying deposition of dust upon the filterelements of the various tanks, an unequal distribution of the flue gasthat is to be cleaned to the various tanks is obtained. If now the tankswere to be cleaned independently of one another and after one another intime, the danger exists that due to the increased flow velocity throughthat tank that has just been cleaned, the pressure differentialattributed to the pressure differential of the untreated and clean gaschannels, the distributor, and the collector, increases unnecessarily.

With the simultaneous cleaning of the filter tanks, a uniformdistribution of the dust-laden gases to the filter tanks, and hence thelowest possible velocity, is achieved. The increase in pressure loss isthen caused only by the pressure differential that is attributed to thefilter elements.

It is furthermore advantageous if the pressure gas pulse is applied tothe interior of the individual filter elements via a jet or nozzle thatopens in the vicinity of the outlet opening of the filter element orextends into this filter element.

It is furthermore preferred that a jet be used that widens in adiffuser-like manner in a direction toward the outlet opening of thefilter element, this jet preferably being one that widens exponentially.

The use of a jet prevents the pressure pulse from expanding sphericallyfrom the discharge opening provided on the previous discharge tube;instead, the pressure pulse is introduced in a directed manner into thefilter element.

BRIEF DESCRIPTION OF THE DRAWINGS

The method of the present invention will now be described in detail withthe aid of the accompanying drawings, in which:

FIG. 1 is a schematic longitudinal cross-sectional view through apressure tank that is equipped with filter elements, including thosegauges or measuring points that are necessary for carrying out themethod with, for example, a charged fluidized bed reactor,

FIG. 2 is a schematic view to explain a preferred preparation of thecleaning or removal pressure for several pressure tanks,

FIG. 3 is a schematic view to explain the cleaning or removal operationwith a plurality of tanks that are supplied in parallel,

FIG. 4 is a partial view comparable to FIG. 1 showing jets or nozzlesprovided on the discharge tubes, and

FIG. 5 is a view similar to FIG. 4 with a jet or nozzle that is widenedin the manner of a diffuser.

DESCRIPTION OF PREFERRED EMBODIMENTS

With a pressurized fluidized bed, a number of filter elements 2 aredisposed in a pressure tank 1 in such a way that the filter elementsreceive from the outside, via a line 3, a flow of dust-laden flue gasthat is under a pressure (p₁). Although the flue gas passes into theinterior of the filter elements 2, the dust remains as a layer S on theoutside of the filter elements. The cleaned flue gas, which is under alesser pressure (p₂), flows into a collection chamber and leaves thetank via a clean gas line 4.

After a certain layer thickness has built up, the filter elements mustbe cleaned off. This is effected by subjecting the interior of thefilter elements 2 to a pressure pulse.

To simplify illustration, a cleaning apparatus is shown associated onlywith that filter element 2 that is illustrated to the right in FIG. 1.This cleaning apparatus comprises a compressed or pressurized gas tube 5that has a discharge opening 5a which is directed toward the interior ofthe filter element. The pressurized gas tube is connected via a solenoidvalve 6 to a compressed or pressurized gas tank 7, which is under apressure (p₃).

If the solenoid valve 6 is opened, a pressure pulse is introduced intothe interior of the filter element that is of such an intensity that thedust cake on the outside of the filter element is removed. The intensityof the pressure pulse must be great enough that the cohesive force ofthe dust layer, and the pressure differential between (p₁) and (p₂) thatis directed against the pressure pulse, are overcome. The dust that isremoved is withdrawn via the dust discharge 8.

As can be seen from FIG. 1, the pressure differential Δp_(a) =p₁ -p₂ ismeasured by a pressure differential indicator 9, and the correspondingpressure signal is fed to a control and regulating unit 10. Furthermore,via a pressure differential indicator 11, the differential pressure ismeasured over a restrictor 12 that is disposed in the clean gas line 4.This differential pressure Δp_(b) is a measure for the flow rate V ofthe flue gas that flows through the pressure tank 1 in conformity withthe following equation ##EQU3## The output signal of the differentialpressure indicator 11 is also fed to the control and regulating unit 10,to which is further fed an experimentally determined lower limitingvalue as a theoretical value S₁. If during the operation the ratio ofΔp_(S) /V_(RG) reaches this limiting value, the solenoid valve 6 isactivated.

In order as a function of the pressure differential Δp_(a) to be able tosubject the filter element 2 to a cleaning pressure pulse of adequateintensity, the tank pressure (p₃) is measured by a measuring device orindicator 13. The control and regulating unit 10 forms the ratio betweenthe pressure (p₃) and the outer pressure (p₁) of the filter elements,and compares this ratio with an experimentally determined theoreticalvalue S₂ that is transmitted to the control and regulating unit. If thepressure (p₃) is inadequate, the tank 7 is furnished with a greaterpressure from a pressure source, which is not illustrated in FIG. 1, viaa pressure-regulating valve 14. Thus, triggering of the solenoid valve 6is preferably effected only when the pressure ratio p₃ /p₁ has reachedthe experimentally determined value S₂.

FIG. 2 schematically illustrates how the cleaning pressure supply tanks7 of a plurality of pressure tanks 1 can be supplied with the requiredcleaning pressure from a main tank 15. By means of a compressor 16, thetank 15 is held at a pressure (p₄) that suffices for setting allpossible pressure values (p₃). Each of the pressure tanks 1 is providedwith the network illustrated in FIG. 1.

The operation of the measured values and the transmitted theoreticalvalues in the control and regulating unit in the context of the presentinvention need not be described in greater detail since such operationsare known.

The pressure differential across the filter elements 2 can be increasedby a pressure fan disposed upstream of the line 3 or by an induced-draftblower disposed downstream of the clean gas line 4. This concerns onlythe increase of the pressure differential across the filter elements.

FIG. 3 illustrates a cleaning apparatus having three tanks 1₁, 1₂, 1₃that are supplied in parallel by the flue gas RG that is to be cleaned.Associated with the tanks are the measuring devices or indicators 9 and11, and corresponding pressure signals Δp_(ai) and Δp_(bi), where i=1,2, and 3, are fed to the control and regulating unit 10. The solenoidvalves 6₁, 6₂, and 6₃ can be controlled by this unit.

From the pressure differentials fed to it, the control and regulatingunit continuously determines the ratios Δp_(ai) /V_(i), where i=1, 2,and 3, and compares these ratios with a limiting value S_(i) that isprescribed for the individual tanks. If the tanks are essentiallyidentical in construction, the determined limiting values S_(i) areessentially equal, so that a common limiting value S can be prescribedfor all of the tanks. If one of the ratios reaches this limiting value,all of the solenoid valves 6₁, 6₂, and 6₃ are activated, so that thetanks are simultaneously cleaned.

Whereas in FIG. 1 illustrated merely schematically is only a singlecompressed or pressurized gas tube 5 with one discharge opening 5a thatis directed toward the interior of the filter element, FIGS. 4 and 5illustrate a compressed or pressurized gas tube 5 that extends overseveral filter elements 2. In the embodiment illustrated in FIG. 4,several linear jets or nozzles 17 are connected to the pressurized gastube 5, with the discharge opening 17a being disposed in the vicinity ofthe outlet opening of the filter element 2. The jets 17 can also extendinto the individual filter elements. This embodiment prevents theformation of a spherical pressure pulse wave as occurs at the opening 5aof FIG. 1; in other words, with this embodiment the cleaning pulse istransmitted better into the individual filter element.

In the embodiment illustrated in FIG. 5, connected with the compressedor pressurized gas tube are individual jets or nozzles 18 that areembodied in a diffuser-like manner and that are each also associatedwith a filter element 2. Also with this embodiment of the jets is abetter utilization of the cleaning pulse achieved.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

We claim:
 1. In a method of removing dust from a dust-laden gas usinggas-permeable filter elements that are disposed in at least one tank,with said gas flowing through said filter elements from the outside tothe inside thereof and depositing said dust on the outer surfaces ofsaid filter elements, and with said deposited dust being removed fromsaid filter elements by pressurized gas cleaning pulses that are appliedto the interior of said filter elements, whereby the pressuredifferential that occurs across the filter elements is measured and, forsaid tank, in addition to said pressure differential across said filterelements, the flow rate of the dust-laden gas is determined, and thecleaning pulse is released when the ratio of the pressure differentialto the flow rate, which ratio increases with time during the formationof a dust layer on said filter elements, reaches at least one limitingvalue, the improvement wherein:a plurality of tanks are supplied inparallel with partial streams of said dust-laden gas; the ratios of thepressure differentials across said filter elements, to the flow rates,of the individual partial streams, which ratios increase with timeduring the formation of dust layers on said filter elements, aredetermined; and when the determined ratio of one of said tanks reachesan experimentally determined limiting value, this tank and all of theother tanks are simultaneously cleaned by pressurized gas cleaningpulses that are supplied to the interior of the individual filterelements.
 2. A method according to claim 1, which includes the step ofsetting the pressure of said cleaning gas as a function of said pressuredifferentials that are obtained across said filter elements in such away that the intensity of said cleaning pulses is sufficient to effectcleaning.
 3. A method according to claim 1, which includes the step ofproviding jet means to apply said pressurized gas pulses directly to theinterior of the individual filter elements.
 4. A method according toclaim 3, in which said jet means open out in the vicinity of outletopenings of said filter elements.
 5. A method according to claim 3, inwhich said jet means extend into said filter elements.
 6. A methodaccording to claim 3, in which said jet means widen in a diffuser-likemanner in a direction toward outlet openings of said filter elements. 7.A method according to claim 3, which includes the step of providing jetmeans to apply said pressurized gas pulses directly to the interior ofeach of the individual filter elements.